The present invention relates to a horizontal focus circuit in an image display using a cathode ray tube (CRT), and more particularly to a horizontal focus circuit which can improve the focus of an electron beam scanned at both edges of the screen of a planar cathode ray tube.
Generally, an image display such as a television receiver, a monitor, etc. displays an image using a cathode ray tube. Such an image display comprises a horizontal focus circuit for controlling the focus of an electron beam scanned on the CRT screen. In conventional art, the CRT screen has changed in recent years from a spherical shape as shown in FIG. 1A to a planar shape as shown in FIG. 1B. The image display drives the CRT by various sync signals having different frequencies. The horizontal focus circuit should generate an output signal whose frequency and amplitude characteristics vary according to the screen shape of such a CRT and the frequency of the sync signal. However, since the conventional horizontal focus circuit has a single frequency and amplitude characteristics, it has the problem of not being applicable to a planar CRT or a CRT having a different number of scanning lines. The above-mentioned problem will be described with reference to FIG. 2, which shows a conventional horizontal focus circuit, and to FIGS. 3A to 3F, which show output waveform diagrams for the several portions of the circuit shown in FIG. 2.
Referring to FIG. 2, a horizontal deflection circuit 10 processes a horizontal sync signal applied via an input terminal 5 to generate a deflection driving signal as shown in FIG. 3A. A transistor Q1 inverts the deflection driving signal applied to its base through a rsistor R1 from the horizontal deflection circuit 10, yielding the signal shown in FIG. 3B. A first transformer T1 inverts and boosts the deflection driving signal inverted by the transistor Q1, as and supplies the boosted deflection driving signal shown in FIG. 3C, to the base of a transistor Q2 through a filter composed of two resistors R3 and R4 and a capacitor C3. The transistor Q2 inverts the signal filtered by the filter and supplies a flyback pulse, as shown in FIG. 3D, to a second transformer T2. The second transformer T2 then generates a deflection output from the flyback pulse supplied from the transistor Q2 and applies it to a deflecting yoke HDY through a serial circuit composed of a resistor R6 and a capacitor C5, and a coil C1 connected in parallel with the serial circuit. A capacitor C6 connected in series with the deflecting yoke HDY integrates the deflection signal applied to the deflection yoke HDY, rendering it a parabola signal as shown in FIG. 3E, and applies the parabola signal to a third transformer T3 through a resistor R7 and a capacitor C7. The third transformer T3 inverts and boosts the parabola signal supplied from the capacitor C7, and applies the (inverted) boosted parabola signal, shown in FIG. 3F, to a focusing grid of the CRT through the resistor R7 and a capacitor C8.
The conventional horizontal focus circuit integrates the deflection signal supplied by the capacitor C6 and supplies the integrated parabolic signal to the focusing grid, thereby controlling the focus of electron beam at the edges of the conventional spherical CRT screen. However, when the conventional horizontal focus circuit is applied to a planar CRT and a multi-sync monitor having various sync frequencies, it is not suitable and there is a problem in that the electron beam is defocused at the edges of the screen.